1. Field of the Invention
The present invention relates to a zoom lens system of a compact camera, and in particular, to attaining a high zoom ratio and miniaturization of the zoom lens system.
2. Description of the Related Art
A zoom lens system for a compact camera does not require a long back focal distance, unlike a zoom lens system for a single lens reflex (SLR) camera in which a space to provide a mirror behind the photographing lens system is required. Accordingly, in a compact camera, a telephoto type lens system including a positive lens group and a negative lens group, in this order from the object, has been employed, while in a SLR camera, a retrofocus type lens system including a negative lens group and a positive lens group, in this order from the object, has been employed. In the above two-lens-group conventional zoom lens system for a compact camera, if an attempt is made to obtain a zoom ratio of about 3.5, it has been difficult to correct aberrations, in particular, curvature of field. Therefore a three-lens-group zoom lens system in which each lens group is arranged to move independently upon zooming has been employed. In such a three-lens-group zoom lens system, a high zoom ration is obtained by moving all the three lens groups. For example, in the case where a three-lens-group zoom lens system including a positive lens group, a positive lens group and a negative lens group, in this order from the object, is provided, the distance between the first and second lens groups has to be made long in order to obtain a high zoom ratio. However, if an attempt is made to obtain a zoom ratio of about 3.5 in the above conventional positive-positive-negative zoom lens system, the distance, at the short focal length extremity, between the most object-side surface of the first lens group and the most image-side surface of the second lens group becomes long, and the diameter of the most object-side lens element in the first lens group has to be made large in order to maintain light amount at and around the short focal length extremity. An increase of the diameter of the most object-side lens element is quite disadvantageous for miniaturization of a camera.
It is an object of the present invention to provide a miniaturized three-lens-group zoom lens system which (i) has a zoom ratio of about 3.5, and (ii) can reduce the diameter of the most object-side lens element in the most object-side lens group, though the zoom lens system is the three-lens-group arrangement.
In order to achieve the above-mentioned object, there is provided a zoom lens system including a negative first lens group, a positive second lens group, and a negative third lens group, in this order from the object. The combined power of the negative first lens group and the positive second lens group is positive. In the zoom lens system, upon zooming from the short focal length extremity towards the long focal length extremity, all the three lens groups are arranged to be monotonously moved from the image side towards the object side. Further, the zoom lens system satisfies the following conditions:
0.01 less than (d12Wxe2x88x92d12T)/fW less than 0.1xe2x80x83xe2x80x83(1)
3.5 less than fT/f12W less than 6.0xe2x80x83xe2x80x83(2)
4.0 less than fT/|f3G|8.0xe2x80x83xe2x80x83(3)
wherein
d12W designates the distance between the first lens group and the second lens group at the short focal length extremity;
d12T designates the distance between the first lens group and the second lens group at the long focal length extremity;
fW designates the focal length of the entire zoom lens system at the short focal length extremity;
fT designates the focal length of the entire zoom lens system at the long focal length extremity;
f12W designates the combined focal length of the first lens group and the second lens group at the short focal length extremity; and
f3G designates the focal length of the third lens group.
The zoom lens system according to the present invention can constitute a telephoto type zoom lens system which has a positive front lens group and a negative rear lens group. This is because both the first lens group and the second lens group can be considered to be a front lens group due to the above-explained positive combined power thereof, and the negative third lens group can be considered to be the rear lens group.
The zoom lens system of the present invention preferably satisfies the following condition:
0 less than logeZ12/logeZ less than 0.07xe2x80x83xe2x80x83(4)
wherein
Z12=f12 T/f12W;
Z=fT/fW; and
f12T designates the combined focal length of the first lens group and the second lens group at the long focal length extremity.
The zoom lens system of the present invention preferably satisfies the following condition:
xe2x88x922.5 less than fT/f3G less than xe2x88x920.5xe2x80x83xe2x80x83(5)
wherein
f1G designates the focal length of the first lens group.
For example, the first lens group can be constituted by a positive first sub-lens group and a negative second sub-lens group, in this order from the object. In this case, preferably, the first sub-lens group includes a positive single lens element, and satisfies the following condition:
1.4 less than fT/f1P less than 4.0xe2x80x83xe2x80x83(6)
wherein
f1P designates the focal length of the first sub-lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-11-371587 (filed on Dec. 27, 1999) which is expressly incorporated herein by reference in its entirety.